The present invention relates to an eyelet buttonhole sewing machine.
It relates more specifically to an improvement in a sewing machine of a type having a table driven by two motors in two directions and receiving sewing material, having sewing implements, and having a cutting device for producing, in the sewing material, a buttonhole incision. The incision is delimited by zigzag stitches of a buttonhole bead which run around the incision. The incision is produced either before (in the "precutting mode") or after (in the "postcutting mode") the buttonhole bead. The sewing implements comprise a needle bar, driven up and down and oscillating additionally in the horizontal direction, and a needle, which is provided at the lower end of the needle bar and which cooperates with a looper mounted in the baseplate.
In the precutting mode, the sewing material is first incised and the buttonhole bead is then produced around the incision. In the postcutting mode, the buttonhole bead is produced first and then the sewing material is incised. In the postcutting mode, an interspace (cutting space) is formed between the two mutually opposite stitched rows forming the buttonhole bead, so that, when the buttonhole is subsequently cut, only the sewing material is cut, and not the sewn buttonhole bead. In the precutting mode, the mutually opposite stitch rows are formed exactly next to one another, so that the precut cloth is prevented by the stitches from fraying.
A sewing machine of this general type is disclosed, for example, in DE 33 02 385 A1, which is equivalent to U.S. Pat. No. 4,501,207. The motors used are stepper motors, so that this sewing machine can be controlled digitally. By means of a suitable control program, the table movement can be controlled so that different buttonhole shapes can be cut and sewn or embroidered. There is no need for complicated cam mechanisms in order to produce the buttonhole bead in either the precutting or the postcutting mode. In this sewing machine, the sewing implements can be rotated by a third motor. Different buttonhole shapes are stored in a control device, and are correspondingly retrievable therefrom.
U.S. Pat. No. 1,991,627 discloses an eyelet buttonhole sewing machine, in which the needle bar is deflected in the horizontal direction by an oscillating shaft, coupled to the arm shaft via a gear, in order to produce the zigzag stitches.
DE 41 32 586 C2, which is equivalent to U.S. Pat. No. 5,125,349 discloses an eyelet buttonhole sewing machine, in which the intermediate material, which is left within the buttonhole bead in the postcutting mode, is provided by shifting the sewing material in order to produce a so-called offset. This offset is imparted to the transport table, which is driven by two stepper motors arranged in axes (x, y) perpendicular to one another. That is to say, before beginning stitch formation, in order to form one side of the buttonhole bead, the transport table is brought into such a position that the inner needle stitch provided on that side is an appropriate distance from the inner stitch on the opposite side of the buttonhole bead. By the storage of different data records, according to which the stepper motors are controlled, the offset can, depending on the working mode, be provided by a corresponding displacement of the table.
DE 21 54 515 C2 (which is equivalent to U.S. Pat. No. 3,656,443) discloses a sewing machine which can be provided with a special presser foot in order to produce a simple buttonhole. The model for the buttonhole to be sewn is a button of the size for which the buttonhole is to be sewn. The oscillating range of the needle bar can be set according to the desired stitch width, and, in order to produce the buttonhole, the button size is sensed mechanically and the sewing material is guided correspondingly by the special presser foot in a closed curved profile.
All the buttonhole sewing machines described above must have a wide needle hole in the throat plate, designed according to the side-to-side or "swing-out" movement of the needle, so that zigzag stitches can be sewn for forming the buttonhole bead. Due to the wide needle hole, it is, of course, not possible for the needle to be guided accurately while penetrating into the sewing material, so in the above machines the stitch pattern cannot have an optimal appearance.
Moreover, the loop catchers allow only limited stitch widths. Depending on the shape of the desired buttonhole, therefore, different sewing implements (such as a threaded looper, a non-threaded looper and a looper spreader, each associated with a looper and if appropriate also the throat plate) must be installed and adjusted in the sewing machine, thus leading to considerable changeover times. An example of such prior art looper spreaders or spreaders, used to spread a loop of thread to permit entrance of the descending needle, is found in U.S. Pat. No. 2,020,779.
In view of these problems, it is desired to improve the known sewing machine so as to increase the universality of use of the machine. In particular, any desired stitch width should be obtainable, without having to exchange the sewing implements, and, at the same time, unimpaired optimal stitch formation should be achieved.